1. Field of the Invention
Embodiments of the invention relate to DC-DC converters and, more particularly, to current limit schemes for such converters.
2. Description of the Related Art
DC-DC converters are a class of power converter. They are used to convert a direct current (DC) signal from one voltage level to another. These converters are commonly used in portable electronic devices that are powered by batteries, such as laptops and cellular phones. DC-DC converters are particularly useful in applications that have several different sub-systems, requiring several different voltage input levels.
There are several different schemes for DC-DC conversion. Linear regulators convert an input voltage to a lower output voltage by dissipating power through thermal radiation. For larger voltage drop high-current applications, these devices are inefficient and, thus, rarely used. A more commonly used scheme is switched-mode conversion. Switch-mode converters convert voltages by periodically storing energy in inductive and/or capacitive components and then releasing that energy to produce the desired voltage level. Inductive components store energy in the form of a magnetic field; whereas, capacitive components store energy in an electric field.
DC-DC converters that use a magnetic energy storage mechanism comprise inductors or transformers. The output voltage is controlled by modulating the duty cycle of the voltages used to charge the inductive component. One common type of magnetic storage DC-DC converter is the buck converter which is a well-known step-down converter, meaning that the regulated output voltage is always less than the input voltage.
FIG. 1a and FIG. 1b are circuit diagrams of a typical buck converter 100. Energy is periodically stored in an inductor L and then released to the load. During each periodic cycle, two switches SWH and SWL are used to alternately connect one end of inductor L to input source VIN during the charge phase and to ground during the discharge phase. When the high side switch SWH is closed (shown in FIG. 1a), current through the inductor L (IL) rises linearly, charging the inductor L. Then SWH is opened and the low side switch SWL is closed (shown in FIG. 1b), and IL decreases linearly, discharging the inductor into the load. As the inductor L is discharging, IL decreases but still flows in the same direction into the load because the stored magnetic energy prevents the current through the inductor from changing direction instantaneously. The switches are turned on and off periodically at a fixed frequency such that the duty cycle determines the ratio of output voltage to input voltage.
One challenge associated with buck converters is protecting the circuit in a current-limit event, such as a short circuit at the output. If the output voltage VOUT suddenly goes to zero, IL will rise rapidly and eventually saturate the inductor L. Prolonged saturation will damage the circuits. Several known schemes have been employed to detect a current limit event and protect the system from runaway current damage by limiting the current through the inductor.
In order to detect a current limit event, the current through the inductor must be sensed. Sensing can be done across the inductor itself; however, this requires external components and a current sensing amplifier with a wide common mode range. The inductor current can also be sensed through the high side switch. This is problematic though, because this scheme also requires a current sensing amplifier with a high common mode range as well as a long settling time, resulting in a restrictive minimum on-time. Finally, the inductor current may be sensed through the low side switch during the discharge phase. Such a scheme is embodied in products manufactured by Analog Devices, such as ADP2114 and ADP1877.